Orthicon electrode structure



May 14, 1957 A. A. RoTow ETAL ORTHICON ELECTRODE STRUCTURE Filed Dec. 7, 1950 AI. ,r llllllllll n.

f n r s l f i I Illll il N /A/f/vmw .llamdcraw @ri Janes ORNEY QRTMCON ELECTRDE STRUCTURE Alexander A. Rotow and Robert B. Janes, Lancaster, ira.,

assignors to Radio Corporation of America, a corporation of Delaware Application December 7, 3550, Seriai No. 199,691

6 Claims. (Ci. 313-55) This invention is directed to an electron discharge device and more particularly to a camera or pickup tube in which an optical image is converted to an electric signal.

The invention is directed to an improvement in an image orthicon type camera tube. Such ya tube is one having an image section including a photocathode electrode upon which a scene to be televised is projected. Photoelectrons from the photocathode are focused upon an insidator target electrode to establish a charge pattern on the target corresponding to the optical image focused on the photocathode. An opposite surface of the target is scanned by a low velocity electron beam to discharge the charge pattern on the target surface as well 'as to provide modulation of the reiected portion of the scanning 'beam to provide a video television signal. The image orthiconcamera tube is well known and is fully described in U. S. patent to H. B. Law, 2,460,093.

ln the `operation of the image orthicon pickup tube, there is produced a high light flare known as a ghost, which consists of a spurious image of high light objects on a dark background. Such spurious images appear on the transmitted scene adjacent the object itself and at a distance from it. The ghost depends upon the location of the high light spot and the conditions under which the image orthicon is operated. This are is noticeable if a white or very light colored object, located with a very dark background, is intensely illuminated.

The invention is specifically incorporated in means for preventing ghosts or spurious images from being transmitted by an image orthicon pickup tube. in `the operation of the image orthicon pickup tube, the photoelectrons from the photocathode surface are accelerated to relatively high velocities and strike the insulator surface of the target electrode with suiiicient energy to produce a secondary electron emission from the target electrode. The secondary electrons are of varying velocities and will partially pass to a collecting mesh electrode closely spaced from the target surface or will be redistributed to other positive portions of the target surface. Within the image section, an electrostatic held is established between the photoelectric cathode and the target electrode to accelerate the photoelectrons toward the target surface. A coil, however, is used to provide a magnetic field, extending between the photocathode and the target surface of the image section, for focusing the photoelectrons onto the target. lt has =been found that due to the mount structure of the target assembly, the eects of the electrostatic and magnetic fields on the photoelectrons within the region immediately in front of the target electrode are not cancelled out and tend to produce a transverse displacement of secondary electrons leaving the target surface. Secondary electrons which are redistributed on the target surface consist of both high velocity and low velocity electrons. The low velocity electrons will fall back essentially on the same portions of the target surface from which they left. However, the high energy electrons will pass through the collector mesh toV a point Patented May 14, 1957 where they are turned about and reiiected back to the target surface. If the effects produced -on these electrons vby the electrosotatic and magnetic fields are not cancelled out, the high velocity electrons will not return to the same point on the target surface from which they left. These high velocoioty electrons thus strike another portion of the target surface and give rise to an additional secondary electron emission, which provides va spurious charge on the target surface and results in an unwanted signal in the output of the tube. This signal provides the ghost or spurious image on the transmitted scene. Y

It is therefore an object of our invention to provide means for improving the operation of an image type camera-pickup tube.

It is another object of our invention to eliminate spuri' ous images in the transmitted picture produced from an image type camera tube.

It is a further object of our invention to provide novel structure Within the image section of -a camera tube to prevent spurious charges on the target surface.

It is another object of our invention to provide novel means within the image section of a camera tube to cause the effects of the magnetic and electrostatic fields on the photoelectrons within the region of the target to be cancelled.

The invention is directed to a television pickup tube, in which there is provided two tubular accelerating electrodes between the photocathode and the target electrode having specic dimensional limitations. The tubular accelerating electrodes mounted within the tube farther from the photoca-thode, is formed as a cup having a thin target sheet of glass mounted across an opening in the bottoni of the cup. lt is found that if the ratio of the length of the first accelerating electrode to the length of the second accelerating electrode is between 0.46 and 0.82, the effects of the magnetic and electrostatic iields within the target cup region on the photoelectrons will cancel each other.

The novel features which are believed to lbe characteristic of our invention are set forth with particularity in the appended claims, but the invention itself Will best be understood by reference to the following description taken in connection with the accompanying drawing, in which:

Figure l is a cross-sectional longitudinal view of an image orthicon camera tube incorporating our invention;v

Figure 2 is 'a schematic showing of the eifect produced -by a highly illuminated spot on an image orthicon tube, without our invention;

Figure 3 is a schematic showing of the effect of a highly illuminated spot on a pickup tube.

Figure l shows an image orthicon camera tube cornprising an envelope 10 having an enlarged portion l2 at one end for enclosing an image section tube described. Within the opposite end of the tubular envelope is an electron gun structure 16 comprising conventional heater, cathode and control grid structures (not shown), for producing an electron beam 14. An additional accelerating electrode 2i? is formed as a wall coating on the inner surface of the tube envelope, for accelerating the electron beam 14 toward a target electrode i8. Pairs of horizontal and vertical deflecting coils are formed into a yoke structure 21 surrounding the tube envelope. T he deiiecting coils provide fields perpendicular to each other and to the tube axis. The deiiecting coils are connected, as is well known, to saw-tooth current sources for providing frame and line scansion of the electron beam ld over the surface of the target 1S. Such a deiecting system is well known in the art and is not described in greater detail, as it is not a part of our invention.

A decelerating electrode 22, formed as a ring, is mounted within the envelope immediately in front of' target electrode 18 and on the scanned side thereof. A low potential established on the decelerating electrode brings 3 the velocity of the electron beam to substantially zero, in front of the target surface. Surrounding the tube envelope is a single coil 2li for providing a magnetic field having lines of force parallel to the tube axis and extending from the end of gun structure i5 beyond the end of the envelope portion 12. The 4field ofvcoil 24 provides a focusing action on the electrons of beam i4., to bring them to a small, well defined point `of focus on the surface of target 18.

At the opposite end vof the tube envelope, there is formed a photocathode electrode 25. Such a photocathode surface may be one such as formed from a sensitized alloy of silver and bismuth as set forth in copending application Serial No. 79,328 of R. Ejiohnson, tiled March 3, 1949, now Patent No. 2,682,479.

First and second accelerating electrodes, 2S and re* spectively, are mounted coaxially'to the tube envelope and are closely spaced from the photoelectric surface of cathode 26. These electrodes provide an accelerating .and a converging electrostatic eld in front of the photocathode 26 to urge the photoelectrons therefrom toward the target electrode l at a high velocity and to condense the photoemission to strike the smaller area lof target i8. The voltage of accelerating electrode 23 is Variable so that the accelerating eld between electrodes 23 and 3i) can be adjusted to eliminate distortion of the picture. Target electrode 18 is formed of an insulator such `as a thin hlm of glass having a slight conductivity and made in the manner set forth in U. S. Patent 2,473,220 to Albert Rose. Photoelectrons from photocathode 26 will strike the adjacent surface of the glass lm 18 with suicient energy to provide a secondary emission therefrom greater than unity.

A fine mesh screen 3d is closely spaced from the photocathode side of the glass hlm 18. Screen 34 serves as a collector electrode and prevents the glass target surface from charging to -a potential higher than that of the screen 34. Glass film 1S is mounted on a ring 19 fixed `adjacent to a short tubular mounting member 36, to which the metal mesh screen 34 is also attached intermediate the ends of member 36. As indicated in Figures l and 2, mounting member 36 is a restricted or hanged portion of the second accelerating electrode 3G. The potential of electrode 3i) and thus mesh 3d is maintained at several volts positive relative to the potential of the cathode of electron gun 16. The use of a iianged member 36 in tubes of this type is to shield target sheet 13 and mesh 34 from metal evaporated during formation of the photo cathode surface 25. As set forth in the above copending application to R. E. Johnson, ilamentary evaporators are mounted in the channel between the wall of electrode and the hanged mounting member 36. During tube processing, the photocathode 26 is formed by evaporating a silver bismuth alloy onto the end of the tube envelope 12. During this step it is necessary that the evaporated metal alloy does not fall on the surfacev of target sheet 1S. The flanged mounting member prevents this.

' The operation of the tube of Figure l is brieily 'as follows. With no illumination on photocathode 2o, electron beam i4 is scanned across the target surface. Low energy electrons from the beam land on the target surface `and drive the surface to substantially zero or gun cathode potential. At this point, the remaining electrons of the beam are reflected toward the gun l5. When `a light pattern is focused on the photocathode 2e, photoelectrons are emitted from each illuminated portion of the photocathode in an amount proportional to the light intensity thereon. The photoel'ectro-ns strike the surface of insulator glass target sheet 1S and initiate a secondary emission from the bombardedV areas to drive them in 1a positive direction, toward the potential yof collector screen 34. ln Vthis manner, there is set up on the photocathode side of the glass film i8 a charge pattern corresponding to the pattern of light `or illumination .focused upon the phot-ocathode '26. Due to the extreme thinness of the glass targetsheet 18, there is established a potential pattern on the scanned side of hlm i8 corresponding to the charge pattern on the photoelectric side of the target. Accordingly, the potential of the scanned surface of target it', will vary from point to point, from substantially zero volts to several volts positive up to the potential of collector screen 34.

The electron beam r4 will Vapproach target electrode i 18 at very low velocity immediately in front of the target surface. When the beam approaches target areas which are at zero potential, it is reflected back toward the electron gun i6. However, more positive areas of the target surface will cause electrons from'the approaching beam to land in numbers suflicient to neutralize the positive potential charge at the respective target area, and thus, drive the charged area of the target to cathode potential. The remaining electrons of the beam are then reflected back to the gun end of the tube. In this manner, then, as the electron beam is scanned over the target surface, there is reflected toward the gun end of the tube a modulated return beam i4. The return beam follows substantially the same path asV the incident beam 14 and strikes the end 17 of the gun structure which is formed as a dynode electrode and as the first stage of a multiplier section 4i). The multiplier 40 is of a type disclosed in U. S. Patent 2,433,941 to P. K. Weimer. The modulated electron beam i4 is converted, asis well known, into output video signal voltages from the collector electrode 42 of the multiplier section 413.

In tubes of the type described above, an effect is produced of a high light are or ghost, which can be seen on the transmitted picture as a spurious image somewhat displaced from the actual image of the transmitted high light. This flare is noticeable particularly if a white or highly illuminated object of light color is located on or before a very dark background. Thus, if a televised scene consists of a black screen with highly illuminated windows in it, or of for example, a man in black clothes having a white shirt, or a White handkerchief in his pocket, the scene is transmitted by the tube, but there is also reproduced a high light flare, which is seen beside the transmitted picture and slightly displaced from it. For example, in Figure 3, there is disclosed what would be seen if a pair of White spots on a black background Were transmitted. The transmitted picture would appear as white spots 44 and 46 on a black background 47. But `also there would appear a darker black hallation 53 about the white spots, and also, 4as is shown, a ghost or grayish picture of the white spots, and as indicated at 4S and Si). The resulting picture, in which such spurious images appear, is obviously very undesirable.

Figure 2 shows a detailed View of the image section of a conventional image-type camera tube, in which structures identical to those of Figure l have the same reference numbers. It has been found, that in the` tube described, the accelerating field between the photocathode electrode 26 and the surface of target 18 is substantially that shown in Figure 2, in which the equipotential surfaces of the field are indicated by dotted lines 52. The ,equipotential surfaces 52, as they approach the mounting member 36, dip into the flanged member and are somewhat Vdistorted indicating that the field, immediately iu front of the target surface 18, is not uniform. The electrostatic lines of force of this field are normal to the equipotential Surfaces and are represented schematically by the pair of lines 54 shown in Figure 2. Also, within this region, immediately in front of the surface of glass target 18, are the eld lines produced by the magnetic coil 24. These iield lines are indicated at 56 and run somewhat in the general direction as the electrostatic field lines 54.

However, it is apparent from Figure 2 that the electrostatic field 54 and the magnetic field, 56 immediately iu frontof the target surface 18, are crossed and do not match each other. It has been found that the secondary electrons ejected from the surface of'target' 18 by the photoelectrons have energies varying through a range from several volts to the voltage energies of the photoelectrons. That portion of the low velocity secondary emission, which does not pass to the collector mesh 34, falls back onto the target surface and in the close vicinity of the point from which they left the target surface. These low energy electrons strike the target surface at very low velocity and remain thereon to provide a negatively charged area which in the picture forms the black border 58 around the high lights 44 and 46, shown in Figure 3. This black border or area is a desirable effect since it helps preserve the contrast of the picture at high lights.

The high energy secondary electrons bombarded from the surface or" target 18 tend to pass through the collector mesh 34 until they are reected back to the target surface by the field of accelerating electrodes 28 and 30. Since the high energy secondary electrons travel greater distances, they pass through the crossed magnetic and electrostatic fields represented by field lines 54 and 56 respectively. Because the magnetic and electrostatic fields do not match each other and provide cancelling effects on these electrons, the high energy electrons acquire a radial or transverse velocity depending upon their displacement from the center of the target 1S. in accordance with the laws of electron optics, this radial of velocity component f causes a transverse displacement of the electron trajectories in a plane normal to the magnetic lines 56. Because of this displacement, the high energy secondaries will not fall back onto the target surface at the same point from which they had been emitted but somewhat to the side of the emission point. Also, due to their relatively high energy, these secondary electrons will bombard additional secondary electrons from the target to provide a positive charge pattern on the target surface,

which provides the ghost images in the picture and as shown at 48 and 5G in Figure 3.

In accordance with our invention, the two accelerating electrodes, 28 and 30, are so formed that the resulting magnetic and electrostatic fields more nearly confirm to each other. We accomplish this by providing definite f length ratios between the two electrodes. We have found that if the ratio of the length of tubular accelerating electrode, 28 to the length of tubular electrode 30 is within the range of 0.46 to 0.83, the ghost etect described above is minimized. For the tube described, it appears, that this range of Values is somewhat critical and that if the ratio of the lengths of the electrodes lies outside of this range the ghost eiects are pronounced.

When the ratio of the lengths of these electrodes fall within the range stated above, the accelerating iield between the electrodes is brought closer to the flat surface of grid 34 in a manner that the equipotential surfaces 52. are attened to conform with the plane of grid 34. This attening of the equipotential surfaces of accelerating field $2 more closely aligns the electrostatic field lines 54 with the magnetic eld lines 56 in the region of the glass target sheet 18. Also under this condition, the eEects of the electrostatic and magnetic fields on the high energy secondary electrons from target 18 will at all points nearly cancel each other. Thus, there will be a minimum displacement of the secondary electrons transversely to their paths to andY from the target 1S. This new construction of the accelerating electrodes provides an optimum integrated effect of the fields on the secondary electrons from target 1S.

if the ratio of the lengths of the accelerating electrodes is less than the range set forth above, then the ghost effects will appear due to the accelerating field in the region of the target 18 assuming a curvature as shown in Figure 2. Furthermore, if the ratio of the electrode lengths is greater than the range set forth above, then the accelerating eld on the photocathode side will assume `a curvature which will also displace the high energy secondary electrons transversely to their paths to and from the tar-A get 18.

i In asccessfully operated tube of the type of Figure l, accelerating electrodes corresponding to electrodes 28 and 30 have a diameter of 2.40 inches and the lengths of the accelerating cylinders corresponding to electrodes 28 and 30 were 0.66 inch and 0.80 inch respectively.

The lengths of cylinders 28 and 30 do not vary directly as their diameters in going to larger or smaller tubes. Thetarget-to-photocathode distance is determined by the focussing field of coil 2li. In the above described tube of the type of Figure l, the photocathode-to-target distance is substantially 1.980 inches and the magnetic focussing field strength is around 75 gauss. A shorter photocathode-to-target distance would require a greater focussing eld to maintain focus of the photoelectrons. This in turn would necessitate a heavier focus coil as well as heavier deflection coils corresponding to yoke 21. A longer photocathode-to-target distance requires a weaker magnetic field to maintain beam focus. However, a weaker focussing eld provides poorer focus and hence there is loss of picture resolution. The photocathode-totarget distance and the field strength described above are optimum values and represent the best compromise between picture resolution and low scanning power for the specific tube described.

For the tube described, the length of the target cup is kept within the limits 0.800 inch and 1.000 inch. The length of the accelerator electrode corresponding to electrode 28 is kept Within the limits 0.660 inch and 0.460 inch.

lt also has been found that the spacing between electrodes 28 and 3i) has an optimum value in the range of 30 to 90 mils. Spacing these electrodes farther than the optimum amount has the effect of moving the center of the accelerating lens, between electrodes 28 and 30, farther from the surface of target 18. This will result in adding curvature to the field surfaces in front of target 18. A space less than 40 mils between these electrodes increases the di'lculty of insulating one from the other.

The ratio of the electrode lengths has been given above as lying in a range between 0.46 and 0.83. However, for optimum results this range of values should be 0.50 to 0.80.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein Without departing from the spirit and scope of the invention.

What is claimed is:

l. An electron discharge device comprising, a photoelectric cathode electrode, an insulator target spaced from said photocathode electrode, a first accelerating tubular electrode between said photoelectric cathode and said insulator target for maintaining an electrostatic accelerating field therebetween for urging photoelectrons from said photocathode to said target, a second accelerating tubular electrode between said rst accelerating electrode and said insulator target for providing an electrostatic converging eld between said first and second accelerating electrodes, said second accelerating electrode including a tubular mounting member fixed at the end of said second accelerating electrode adjacent said target electrode, said target electrode being mounted across the adjacent end of said tubular member, a magnetic coil surrounding the space between said photoelectric cathode and said insulator target for maintaining an electron focusing magnetic field therebetween, the lengths of said first and second accelerating electrodes having a ratio lying in the range between O f to 0.83, the diameters of said first and second accelerating electrodes being in the order of 2.4 inches, and said rst and second accelerating electrodes being spaced apart a distance between 30 and mils.

2. An electron discharge device comprising, a photoelectric cathode electrode, an insulator target electrode spaced from said photocathode electrode, a first tubular accelerating electrode between said photoelectric cathode and said insulator-'target electrode for maintaining an electrostatic accelerating field therebetween for urging photoelectrons from said photocathode to said target, a second accelerating tubular electrode between said rst accelerating electrode and said insulator target electrode for providing an electrostatic converging field between said first and second accelerating electrodes, said second accelerating electrode including a tubular mounting member at the end of said second accelerating electrode adjacent said target electrode, said target electrode being mounted across the adjacent end of said tubular member, a magnetic coil surrounding the space between said photoelectric cathode and said insulator target electrode for maintaining an electron focusing magnetic field therebetween, a collectorscreen mounted across the opposite end of said tubular mounting mer the lengths of said iirst and Isecond accelerating electrodes having a ratio lying in the range between 0.50 to 0.80, the diameters of said iirst and second accelerating electrodes being the order of 2.4 inches, and said first and second accelerating electrodes spaced apart a distance between 30 and 90 mils. g

3. An electron discharge device comprising, an en velope, an electron gun mounted within one end of said envelope, a photoelectric cathode electrode mounted at an opposite end of said envelope, an insulator target spaced from said photocathode electrode and between said photocathode and said electron gun, a first accelerating tubular electrode between said photoelectric cathode and said insulator target for maintaining an electrostatic ccclerating field therebetween for urging photoelectrons from said photocathode to said target, a second accelerating tubular electrode between said photoelectric cathode and said insulator target for providing an electrostatic converging lield between said rst and second accelerating electrodes, said second accelerating electrode including tubular mounting member fixed at the end of said second accelerating electrode adjacent said target electrode, said target electrode being mounted across the adjacent end of said tubular member, a magnetic coil surrounding the space between said photoelectric cathode and said insulator target for maintaining an electron focusing magnetic field therebetween, the lengths of said first and second accelerating electrodes lhaving a ratio lying in the range between'0-46 to 0.83, the diameters of said irst and second accelerating electrodes being in the order of 2.4 inches .and said first and second accelerating electrodes being spaced apart a distance between 30 and 90 mils.

4. An electron discharge device comprising, a tubular entf-"clope, electron gun mounted within one end of said el a photcelectric cathode electrode mounted at the opposite end said envelope, an insulator target spaced from and between said photoelectiic cathode and said electron gun, a first tubular accelerating electrode between said phot-@electric cathode and said insulator target for maintaining an electrostatic accelerating field therebetween for urging photoelectrons from said photoelectric cathode to said target, a second accelerating tubular electrode between said photoelectric cathode and said insulator target for providing an electrostatic converging field between said first and 'second accelerating electrodes, said second accelerating electrode including a tubular mounting i mber fixed at the end of said second accelerating electrode adjacent said target electrode, said target electrode being mounted across the adiacent end of said tubular member, a magnetic coil surrounding the space between said photoelectric cathode and said insulator target for maintaining an electron focusing magnetic eld therebetween, a collector screen mounted across the opposite end. of said tubular mounting member, the lengths of Vsaid lirstpand second accelerating electrodes having a ratio lying in the range between 0.50 to 0.80, means for scanning the beamV of Said electron gun over the surface of said target electrode, the diameters of said first and second accelerating electrodes being in the order of 2.4 inches, and said first and second accelerating electrodes being spaced apart a distance between 30 and 90 mils.

5. A television pickup tube comprising, an evacuated envelope, a photoeleetric cathode electrode within said envelope, an insulator target within said envelope and spaced from said photocathode electrode, a first accelerating tubular electrode adjacent-to said photoelectric cathode and between saidV photoelectric cathode and said insulator target for maintaining an electrostatic accelerating field therebetween, a second accelerating tubular electrode between said rst accelerating electrode and said insulator target for providing an electrostatic converging field between said first and second accelerating electrodes, said insulator target being mounted across the end of said second accelerating electrode away from said photocathode electrode, and a short tubular electrode member enclosed withinV said second accelerating electrode and extending coaXially from the surface of said insulator target, said tube being adapted to use a magnetic coil surrounding the space between said photoelectric cathode and said insulator target for maintaining an electron focusing magnetic iield therebetween, the axial lengths of said rst and second accelerating tubular electrodes being proportioned to provide an electrostatic field within said short tubular member whose lines of force substantially coincide with the iield lines of said magnetic coil.

6. 'A television pickup tube comprising, an evacuated envelope, a photoelectric cathode electrode within said envelope, an insulator Ytarget within said envelope and spaced from said photocathode electrode, a first accelerating tubular electrode adjacent to said photoelectric cathode and between said 'photoeleetric cathode and said insulator target for maintaining an electrostatic accelerating field therebetween, a second accelerating tubular electrode between said first accelerating electrode and said insulator target for providing an electrostatic converging eld between said rst and second accelerating electrodes, said insulator target being mounted across the end of said second accelerating electrode away from said photocathode electrode, and a short tubular electrode member enclosed within said second accelerating electrode and eX- tending coaxially from the surface of said insulator target, a magnetic coil surrounding the space between said photoelectric cathode and said insulator target for maintaining an electron focusing magnetic iield therebetween, the axial lengths of said first and second accelerating tubular electrodes being proportioned with their lengths having a ratio lying in the range between V0.46 and 0.83 to provide an electrostatic field within said short tubular member Whose lines of force substantially coincide with the field lines of said magnetic coil.

References Cited inthe tile of this patent UNITED STATES FATENTS 

